The present invention monitors an induction motor while it is operational, and detects and locates rotor bar faults in real time without interfering with the operation of the motor.
The prior art includes numerous methods of detecting "cage faults". Most require one or more of the following actions: disassembly of the motor, motor shut down, and/or special connection of instrumentation inside the motor. For instance, the growler method uses an electromagnet coupled to the rotor surface which emits a loud noise when it spans an open rotor bar. This requires disassembly of the motor.
Single phase testing requires disconnecting one phase of the motor's power supply, and monitoring the current drawn while exciting the remaining terminals at low voltage and rotating the rotor slowly, by hand. If there is a broken bar the current drawn will vary with rotor position. While sensitivity is good--a broken bar is usually clearly evidenced by a current variation of over five percent--the motor must be taken out of service and one phase disconnected. Further, the low voltage power requirement is considered to be a safety hazard by many utility companies.
Unlike prior art systems, the present invention unambiguously detects rotor faults while the motor is running. Furthermore, it identifies the location of the rotor bar with the fault. The ability to locate the bar or bars at fault is of value because it facilitates fault identification and repair.
As will be described in more detail below, the present invention works by using a coil on a stator tooth, inside the motor, to measure the flux around (actually, in the air gap above) each rotor bar while the motor is running. By synchronizing the measurement process with rotation of the rotor, the flux measurements for each rotor bar can be separately identified. The underlying theory of operation is that the measured flux will be markedly different for normal rotor bars and for rotor bars with faults.
The main disadvantage of the present invention is that it requires the installation of a coil on a stator tooth inside the motor. Clearly, for new or rewound motors this is not a problem, because the coil can be installed when the motor is being assembled. For motors which have already been assembled, and for those already in operation, the disassembly required for installing a stator tooth coil obviously requires an interruption in service. On the other hand, only one interruption is required in the life of the motor because the fault monitoring process itself is performed during normal motor operation.
It is therefore a primary object of the present invention to provide a system and method for detecting and identifying the location of rotor bar faults in induction motors.